In general, the darker the room the less light reflection you will get from the screen and/or lights in the room. Having a dark wall behind the RPTV will help make the screen stand out more and give you an illusion of increased contrast...since your eyes are only gathering light from the screen rather than also from bright or light colored backbrounds...make sense?
I went with black front wall and deep red/blue/black in my theater...you can see pics at my website below.
In short, do what you guys like and can live with...it's about your enjoyment!
E